Ah, no wonder, then. I activated the "high frequency mode" feature in AudioCatalyst when I used it. From the help file:

"High Frequency Mode when selected, enables encoding of high frequencies (up to 20kHz), for CBR bitrates 112k and VBR settings of Low/Normal and above. Note: Only a small percentage of users have both hardware that can clearly reproduce high frequency sounds and exceptional hearing to distinguish those high frequencies from lower frequencies within the music."

Okay, I will revise my statement. When I deactivate the Xing low pass filter, and I encode at a decent bit rate, I can't tell the difference between LAME and Xing.